1. Field of the Invention
A digital modem including circuitry capable of digital manipulation and computation of digitized data signals to accomplish the required encoding, filtering, modulation, demodulation, equalization, decoding and control function.
2. Description of the Prior Art
Existing data communications systems which require an immediate transfer of data between a computer and another distant computer or I/O device, normally use voice-grade telephone lines as the transmission medium between two locations. Systems that have only a small number of I/O locations and a need for nearly 24 hours per day data transfer commonly employ special dedicated or private lines between locations. These lines are leased from common carriers and are often engineered to meet special data signaling requirements.
However, there is a large class of phone-line data transmission users hereafter referred to as telecommunications users which need the switching flexibility of the dial-up telephone network for telecommunications. It is these users which are especially hampered by serious imcompatibilities in existing systems, and for which the present invention provides an effective solution.
To successfully transfer data signals over the bandwidth limited phone network, special signal processing equipment or modems are required. These modems generally accept the DC pulse output from a computer or terminal equipment and convert this pulse to an audio signal suitable for transmission over voice-grade lines or similar facilities. These signals are reconverted to their original pulse state at the receiving end for use by the terminal equipment.
Characteristically these devices employ hardwired logic circuitry specifically designed for particular signals processing functions. Such hardwired circuitry inhibits the wide or general use of specific design configurations. Moreover system integration of components between locations exhibiting different hardwired specifications such as filters and equalizers complicate and compound overall system design and hardware.
Recent advances in digital circuitry provide a means of implementing a programmable digital signal processor or modem with greater flexibility and less cost than that generally associated with the more common hardwired logic block systems. Thus implementation of a programmable signal processor or modem controlled by software-like instructions which manifast characteristics generally associated with digital computers to derive or process analog signals with A/D and D/A converters is impossible with digital manipulation of the digitized signal samples. Moreover digital manipulations or computations are easily altered for system compatibility with user requirements.